This depends greatly on how the prob/stat and discrete math classes are taught. At my college our discrete math for math majors has linear algebra as a prereq (which has calc 2 as a prereq) and is quite intense, while the discrete math for CS majors has no math prereq. The prob and stat are two separate courses at my school, where prob is a prereq to stat, and has calc 3 and linear algebra as a prereq. Probability is considered one of the hardest courses in the entire department.
If you are able to register for all three, then obviously your university has the courses structured differently. Do you have access to the course descriptions? If you could post them, we might have a better idea of how rigorous each of those courses is.
At my university, students generally don’t take more than two math classes at a time.